I think this is required.
But i would do it as follows;

Keep it as 4 subs, and have 2 reserves who could be called upon if there are injured players only.

I can see where you're coming from, but wouldn't this be awkward in that a coach would then be forced to decide on whether a player is good enough to be a replacement or a sub? Also, it would be open to abuse.

What about more subs but less interchanges? It's a better balance I feel.

5 subs, 10 interchanges.

But who decides if a player is injured or not? It's open to abuse like the union blood replacement rules.

Yeah, it'd be difficult to police an injured players system.

I think five subs and 12 interchanges may be a good idea, with two of these subs being 'home-grown' and under the age of 20.

The original poster was concerned more with giving youngsters a chance, I think in line with that point of view it might be better to have your normal 4 subs + 2 younger (under 19?, or under a certain number of games, perhaps?) but still keeping inside the 12 interchanges.